Radiotherapeutic apparatus

ABSTRACT

A radiotherapeutic apparatus comprises a source able to emit a beam of therapeutic radiation along a beam axis, a multi-leaf collimator arranged to collimate the beam to a desired shape, wherein the source is rotateable about a rotation axis that is substantially orthogonal and intersects with the beam axis thereby to describe an arc around that axis, and further comprises a control means able to control the dose/time rate of the source, the rotation speed of the source, and the multi-leaf collimator position. The control means is arranged to receive a treatment plan in which the arc is divided into a plurality of notional arc-segments, and specifying the total dose for the arc-segment and a Start and end MLC position. It then controls the source in accordance with that plan over an first arc-segment such that at least one of the rotation speed and dose rate are constant and the multi-leaf collimator changes shape, and a second arc segment such that at least one of the rotation speed and dose rate are constant at a level different to the constant level adopted during the first arc-segment. It achieves this by calculating the total time required for the arc segment for a plurality of factors including an MLC leaf movement from a prescribed position at the start of the arc-segment to a prescribed position at the end of the arc-segment, at a maximum leaf speed, rotation of the source from the start to the end of the arc-segment at a maximum source rotation speed, delivery of the dose at a maximum dose rate per time, selecting the factor dictating the longest time, and Controlling the apparatus so that the selected factor operates at its respective maximum and the remaining factors are operated at a reduced rate selected to match that longest time, wherein the total time required for the arc segment for at least one factor relating to a moving geometry item is the greater of (a); a time required to complete the segment at a continuous defined upper speed for the geometry item and (b) a time required to accelerate the geometry item until it is travelling at the defined upper speed. Generally, the time required to accelerate the geometry item to the defined upper speed will include a time to accelerate the geometry item to that speed, and a further time to accelerate the geometry item beyond that speed and subsequently decelerate it until travelling at that speed.

This Application is a Section 371 National Stage Application ofInternational Application No. PCT/EP2007/009227, filed Oct. 24, 2007 andpublished as WO 2009/052845 A1 on Apr. 30, 2009, the content of which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to radiotherapeutic apparatus.

BACKGROUND ART

A radiotherapeutic apparatus is typically controlled by a TreatmentControl Computer. When equipped with a Multi-Leaf Collimator (“MLC”) theTreatment Control Computer can be considered to contain a RadiationControl Computer which controls the radiation generation, an MLC Controlcomputer which controls the shape of the MLC and a Gantry ControlComputer which controls the position of the Gantry. These computers mayphysically be one or more computers but in this text are considered asdistinct functional elements of the system. “Mu” is an abbreviation for“monitor units”, which is the term used for units of radiation from theradiotherapeutic apparatus. An mu is equivalent to a unit of dosedelivered to the patient under well defined calibration conditions. Therelationship between mu and dose is modelled in the Treatment planningcomputer. The user interacts with the patient's prescription in units ofdose but the Treatment planning computer defines the Treatment plan inunits of mu. One of the tasks of a Treatment Planning computer is toascertain the mu that need to be delivered by the apparatus in order toachieve a specific dose within the patient, both in terms of asufficiently high dose in the tumour site and a sufficiently low dose inother parts of the patient. Informally, the use of the term ‘dose rate’means ‘mu rate’

Intensity Modulated Radiotherapy is a generic term for a number ofradiotherapy techniques that, essentially, vary the beam that isdirected at the patient. That variation can be spatial, temporal, orboth.

Known linac delivery technologies include the following.

Segmental or Static Multi-Leaf Collimator—“SMLC”—is where the Multi-LeafCollimator (“MLC”) is static during irradiation. The MLC moves from oneshape to the next in between irradiations. In one architecture, thepoint at which the irradiation stops and the MLC moves is controlled bythe dosimetry hardware and Radiation Control computer. This results inexceptionally accurate delivery of dose per MLC shape. An alternativesystem uses a DMLC architecture to achieve the same effect. The MLCControl computer monitors the delivered dose and inhibits radiation whenit detects it should move from one shape to the next. The inevitablecontrol system delays associated with this architecture result in anuncertain dose per MLC shape and occasional missed shapes altogether.

Dynamic MLC—DMLC—is where the MLC moves during irradiation, with thegantry stationary. The MLC moves linearly from one shape to the next asa function of the delivered dose. The MLC control system has to monitorthe delivered dose, and there is an inevitable delay. On older systemsthis delay was 200-300 ms, for more recent systems this is approximately40 to 50 ms. This delay, together the response of the MLC, results inthe shapes lagging behind the dose. This is extensively reported in theliterature, but is widely regarded as not being clinically significant.

Rotational DMLC—RDMLC—is where the MLC moves during irradiation during aconstant rotation of the gantry. The gantry moves at a constant mu perdegree. The MLC moves linearly from one shape to the next as a functionof the delivered dose. The shapes are usually, but not necessarily,defined at regular intervals around the arc. This can be achieved with asubstantially independent MLC, Radiation and Gantry control computers.

Enhanced Rotational DMLC—ERDMLC—is where the MLC moves duringirradiation during a rotation of the gantry and the gantry moves at avariable mu per degree. A variable gantry speed or variable dose rate(or both) can achieve the latter. Using variable dose rate alone hasbeen analysed by the University of Gent as not being the preferredoption as it gives longer delivery times. The MLC moves linearly fromone shape to the next as a function of the delivered dose. The shapesand doses are usually, but not necessarily, defined at regular intervalsaround the arc. This technique requires a very high degree ofintegration between the MLC, Radiation and Gantry control computers and,to date, no linac has been able to deliver ERDMLC. At present, it istherefore a theoretical possibility only.

Treatment techniques involve a compatible treatment planning functionand Linac delivery function, and known techniques are as follows:

Intensity Modulated Radiation Therapy—IMRT—is a sequence of MLC shapeswith associated doses which can be delivered using SMLC and DMLC. Theshapes are defined at a limited number of stationary gantry positions,typically 5 to 9. The shapes and doses are defined by an optimiser whichattempts to meet objectives defined by the user. The treatment planningfunction is generally specific to the MLC constraints and the deliverytechnique.

Rotational Conformal Arc Treatments—RCAT—involves a constant rotation ofthe Gantry while the leaves are fitted dynamically to the projection ofthe target volume. This technique has been in use in Japan for manyyears. The delivery technique is RDMLC and only one arc is used.

Intensity Modulated Arc Therapy—IMAT—involves a treatment planningfunction in which the arcs and the positions of the leaves are notdefined by the projection of the target volume but by an optimisationroutine that tries to deliver the required dose distribution to thetarget and critical structures. In general a number of arcs are usedover different ranges of gantry angles. The optimisation is like IMRTbut includes the added flexibility of the rotational gantry. IMAT can bedelivered via RDMLC, but this imposes a restriction on the optimisationof a constant mu per degree, which results in a sub-optimal plan. Moreideally, the optimisation will be allowed complete freedom and an ERDMLCdelivery technique will be used. The delivery times are exceptionallyquick, typically 3 minutes for a complex plan. Superficially thistechnique looks the same as RCAT but the difference is how the MLCshapes are determined.

IMAT is discussed, for example, in Duthoy et al, “Clinicalimplementation of intensity-modulated arc therapy (IMAT) for rectalcancer”, International Journal of Radiation Oncology, Volume 60, Issue3, 1 Nov. 2004, pp 794-806 which ends “We identified significantpotential for improvements both at the levels of planning and delivery.The single most important technical improvement for IMAT is theimplementation of a variable gantry speed”, i.e. an apparatus capable ofERDMLC.

Optimized Segment-Aperture Mono-Arc Therapy—OSAMAT—is a special class ofIMAT in which only one arc is used. This seems suitable for someclinical indications. It could also be regarded as a refinement of RCAT.Similar to IMAT the delivery technique can be simply RDMLC but moreideally ERDMLC. The delivery times are exceptionally quick, typically 1minute.

Arc Modulation Optimisation Algorithm—AMOA—is the technique used by 3DLine Medical Systems. The leaf shapes are defined by the anatomy (as inRCAT) and then the arcs are divided into smaller sub arcs of about 20degrees and the weight or mu per degree of these sub arcs are optimisedto give the best dose distribution (similar to IMAT or IMRT). Thus, thisis a form of IMAT or OSAMAT in which the option of modifying the leafpositions is not used. This is very quick to plan and to deliver,especially using the ERDMLC delivery technique.

Helical Intensity Modulated Arc Therapy—HIMAT—is a development of theIMAT technique where the patient is translated longitudinallysimultaneously with the gantry rotation. This effectively makes thelongitudinal length of the treatable field unlimited and truly competeswith a Tomotherapy delivery solution. U.S. Pat. No. 5,818,902 andWO97/13552 show details of this. This typically has an MLC in a fixedorientation with the leaves moving across the patient. The MLC can havehigh-resolution leaves and a limited field size, as the field size canbe extended by use of the helical technique.

The delivery technique for HIMAT can be simply RDMLC as the multiplerotations will allow the flexibility of increased dose from certainangles. The delivery times are exceptionally quick, typically 3 minutesfor a complex plan.

Our earlier application PCT/EP2006/003901 proposed a methodology bywhich a close approximation to ERDMLC could be achieved in practice. Thepresent application describes this methodology and explains how it canbe refined to take account of the dynamic properties of the apparatus onwhich it is embodied.

SUMMARY OF THE INVENTION

It is possible that the ERDMLC delivery technique will give advantages,particularly to MAT and HIMAT treatment plans. However, it has notproved possible to deliver ERDMLC in practice. A delivery technologythat approximated to ERDMLC in terms of its capabilities but which wastechnically feasible to deliver would therefore be of great value.

Historically, all arcs have been delivered at a nominally constantrotation speed and constant dose rate, giving a fixed mu per degree ofrotation. This requires constraints to be placed on the treatmentplanning optimisation which degrade the clinical quality of the plan.Further, there is a maximum speed at which the leaves of the multi-leafcollimator can move, and therefore at a given dose rate and dose for anarc segment there is a maximum distance they can travel. This is also aconstraint in the planning, limiting the quality of the plan.

If the optimisation in the Treatment planning computer was to be allowedto vary the mu per degree, then it would put more dose into gantryangles that have fewer critical organs in the path of the radiation. Forexample, when treating the prostate, as the gantry rotates the bladderand rectum come in and out of the path of the radiation. It is notpossible to avoid irradiating these organs completely, nor is itdesirable (otherwise insufficient dose could be deposited into theprostate) but if the optimisation is allowed more flexibility incontrolling the dose to these critical organs then it will be able toreduce the unwanted dose.

If the dose rate for the arc can be lowered, this will allow theplanning more flexibility but increase the time for delivery, which isundesirable. An aim of this invention is to remove such constraints fromthe treatment planning process and therefore maximise the quality of theplan, while at the same time retaining a quick delivery time. Quickdelivery times are important for departmental efficiency and (in highprecision Image Guided Radiation Therapy) to prevent organ motionbetween imaging and the completion of irradiation.

According to the invention, a desired treatment is therefore describedby the Treatment Planning computer in terms of a sequence of “controlpoints”. Each “control point” defines a position of the gantry, the dosethat is to be delivered between this and the next (or previous) controlpoint, and the shape of the MLC at that control point. Each consecutivepair of control points defines (between them) an arc-segment.

This treatment is put into effect by, between the nth and the (n+1)^(th)control point, moving the gantry from the position of the nth controlpoint to the position of the (n+1)^(th) control point at a rotationspeed and a dose rate that combine to deliver the required dose, whilemoving the MLC leaves so that when the gantry is at the (n+1)^(th)control point, the leaves are in the correct position for that point.Typically, the MLC leaves will be moved at a rate which ensures that atall times their distance moved is linearly related to the dose that hasbeen delivered in the arc-segment. This process is then repeated for thearc-segment between the (n+1)^(th) and the (n+2)^(th) control points,and so on until the treatment is complete.

Thus, we propose a radiotherapeutic apparatus comprising a source ableto emit a beam of therapeutic radiation along a beam axis, a multi-leafcollimator arranged to collimate the beam to a desired shape, whereinthe source is rotateable about a rotation axis that is substantiallyorthogonal to and intersects with the beam axis thereby to describe anarc around that axis, further comprising a control means able to controlthe dose/time rate of the source, the rotation speed of the source, andthe multi-leaf collimator position, the control means being arranged toreceive a treatment plan in which the arc is divided into a plurality ofnotional arc-segments, the treatment plan specifying the total dose forthe arc-segment and a start and end MLC position, and to control thesource in accordance with that plan over an first arc-segment such thatat least one of the rotation speed and dose rate are constant and themulti-leaf collimator changes shape, and a second arc segment such thatat least one of the rotation speed and dose rate are constant at a leveldifferent to the constant level adopted during the first arc-segment bycalculating the total time required for the arc segment for a pluralityof factors including an MLC leaf movement from a prescribed position atthe start of the arc-segment to a prescribed position at the end of thearc-segment, at a maximum leaf speed, rotation of the source from thestart to the end of the arc-segment at a maximum source rotation speed,delivery of the dose at a maximum dose rate per time, selecting thefactor dictating the longest time, and controlling the apparatus so thatthe selected factor operates at its respective maximum and the remainingfactors are operated at a reduced rate selected to match that longesttime, wherein the total time required for the arc segment for at leastone factor relating to a moving geometry item is the greater of (a) atime required to complete the segment if the geometry item traveled at acontinuous defined upper speed for the geometry item and (b) a timerequired to allow for adjustment of the speed of the geometry item untilit is travelling at the defined upper speed.

The time required to adjust the speed of the geometry item to thedefined upper speed can be determined in a number of possible ways. Oneoption is to perform a calculation via a suitable physics engine orotherwise from first principles, such as from knowledge of the geometryitem state and knowledge of its dynamic properties. Another is todetermine the time from knowledge of an initial speed and an intendedspeed of the geometry item by taking a proportion of a time toaccelerate the geometry item from rest to the defined upper speed, theproportion being substantially equal to the difference between theinitial speed and the intended speed, divided by the defined upperspeed. A further option is simply to use a preset amount representing atime to accelerate the geometry item from rest to that speed.

The time required to adjust the speed of the geometry item to thedefined upper speed will generally include a time to accelerate thegeometry item to that speed, and a further time to accelerate thegeometry item beyond that speed and subsequently decelerate it untiltravelling at that speed. This can be controlled so as to place thegeometry item on the same distance/time relationship as if it hadchanged to that speed instantaneously.

The control means will typically comprise a treatment control computerand an actuator.

The radiotherapeutic apparatus is preferably arranged to monitor thedose actually delivered during a treatment and the actual position ofthe source and/or MLC, compare this to the treatment plan, and servo theposition of the source/MLC and or the dose rate so that the actualrelationship between delivered dose and source position corresponds tothe treatment plan.

Likewise, the radiotherapeutic apparatus is preferably arranged tomonitor the dose actually delivered during a treatment and the actualposition of the patient positioning system, compare this to thetreatment plan, and servo the position of the patient positioning systemand or the dose rate so that the actual relationship between delivereddose and patient positioning system position corresponds to thetreatment plan.

The radiation is preferably not interrupted between the firstarc-segment and the second arc-segment.

In this way, a system is produced that is sufficiently close to ERDMLCin practice to mean that it can be treated at an ERDMLC system for mostpurposes. This enables us to further propose a treatment planningapparatus for a radiotherapeutic apparatus of the type comprising (i) asource able to (a) emit a beam of therapeutic radiation along a beamaxis and (b) rotate about a rotation axis that is substantiallycoincident with the beam axis, thereby to describe an arc around thataxis, (ii) a multi-leaf collimator arranged to collimate the beam to adesired shape, and (iii) a control means able to control the dose rateof the source, the rotation of the source, and the multi-leafcollimator, the treatment planning apparatus being arranged to dividethe arc into a plurality of notional arc-segments and to prepare atreatment plan which includes a first arc-segment adapted such thatthrough the delivery of a certain number of mu first specified doseduring which the source rotates a certain number of degrees firstspecified angle and the multi-leaf collimator changes shape at a firstspecified rate per degree, and similarly over a second arc segmentadapted to deliver a second specified dose during which the sourcerotates a second specified angle and the multi-leaf collimator changesshape at a second specified rate per degree, such that at least one ofthe first and second specified doses, the first and second specifiedangles, and the first and second specified rates per degree, mu perdegree rotation and the mu per mm MLC leaf movement are constant at alevel different as between the first and second arc-segments.

It is preferred that the beam axis and the axis of rotation of thesource are substantially orthogonal, for reasons of geometricsimplicity.

As will be apparent from the above, we prefer that the rotation speedand the dose rate are both constant during an arc-segment, but that atleast one thereof is different as between the first arc-segment and thesecond arc-segment.

Generally, we intend that the first arc-segment and the secondarc-segment are consecutive. However, there may be specific instanceswhere individual consecutive arc-segments do in fact have the samerotation speed and dose rate. However, in a treatment plan according tothe present invention there will be pairs of arc segments for which atleast one is different.

The treatment planning apparatus will of course include an output meansof some form, for transmitting the treatment plan to theradiotherapeutic apparatus.

Of course, since much of the calculation effort in this method can beconducted on an assumption of movement at a single steady speed, it ispossible to express the various times discussed above in terms of theequivalent distances. Comparisons of the distances are of coursedirectly equivalent to comparison of the associated times.

The treatment planning apparatus can further prescribe a treatment planthat includes motion of a patient positioning system during thetreatment, in a manner correlated with motion of the source and/ordelivery of the dose. This will (inter alia) allow HIMAT treatments tobe provided.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample, with reference to the accompanying figures in which;

FIG. 1 is a graph showing the optimised control points from theTreatment Planning computer as to the leaf position and dose;

FIG. 2 is a graph showing the optimised control points from theTreatment Planning computer as to the gantry position and dose deliveredas treatment progresses, and the approximation imposed by a constant muper degree;

FIG. 3 shows the effect of the control points of FIG. 2, in terms of thedose rate, together with the same approximation;

FIG. 4 shows an ideal calculation of the dose rate (solid line) androtation speed (dashed line);

FIG. 5 shows a practical calculation of the dose rate (solid line) androtation speed (dashed line) in a system without a continuously variabledose rate;

FIG. 6 shows the relationship between the computers;

FIGS. 7 and 8 are graphs of (respectively) speed and distance with time,showing the effect of inertia; and

FIGS. 9 to 11 are bar charts showing the comparison processincorporating inertia compensation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A desired treatment is described by a Treatment Planning computer interms of a sequence of “control points”. Each “control point” defines aposition of the gantry, the dose that is to be delivered between thisand the next (or previous) control point, and the shape of the MLC atthat control point. Each consecutive pair of control points defines(between them) an arc-segment.

Control points could (in theory) be spaced strategically around thecomplete arc. However, the availability of relatively cheap processingpower means that there is little benefit in going to the effort of doingso, and control points are therefore typically spaced regularly aroundthe arc such as every degree, every few degrees, or every fraction of adegree.

Basic Methodology

This treatment is put into effect by, between the nth and the (n+1)^(th)control point, moving the gantry from the position of the nth controlpoint to the position of the (n+1)^(th) control point at a rotationspeed and a dose rate that combine to deliver the required dose, whilemoving the MLC leaves at a substantially constant rate so that when thegantry is at the (n+1)^(th) control point, the leaves are in the correctposition for that point. This process is started at n=1, and thenrepeated for the arc-segment between the (n+1)^(th) and the (n+2)^(th)control points, and so on until the treatment is complete.

Thus, FIGS. 1 and 2 show a pattern of control points for a treatment.FIG. 1 shows graphically the details of the control points in terms ofthe position of a specific MLC leaf as the treatment progresses. Duringthe treatment, tracked in terms of the total mu dose delivered so far,the leaf initially extends, retracts, and subsequently extends again.The dotted line shows the instantaneous position of the leaf, given thatthe control apparatus will move the leaf at a steady rate betweencontrol points so that by the time the next point is reached, the leafis at the desired position. Similar graphs will exist for each of the(typically) 80 leaves; each graph will generally have more than 6control points, such as 45, 90 or 180 control points.

FIG. 2 shows the details of the control points in terms of total dosedelivered as the gantry rotates. Thus, the points are on a monotonicallyrising scale. However, the amount of the increase between successivecontrol points varies, corresponding to some gantry angles at which moreradiation is delivered and some at which less is delivered. The latterwill generally correspond to angles at which the target structure isobscured by a critical structure. The variation in dose delivered can beachieved by variation of either the dose rate per time or the gantryrotation speed, or both. Clearly, a reduction in the cumulative dosedelivered between a range of positions can be achieved by increasing therotation speed or by reducing the dose rate. FIG. 2 shows in a dottedline the approximation that is imposed by requiring a constant mu perdegree; this reduces the flexibility and either requires a less optimaldose distribution, or requires the variation to be taken up by way ofthe MLC positions thereby extending the treatment time.

FIG. 3 shows the result of FIG. 2, in terms of the dose rate at eachgantry angle. At some gantry angles, the dose rate is high, indicatingthat a clear view of the target structure is available. At other gantryangles, the dose rate is markedly reduced indicating that the target maybe obscured by a critical structure.

Thus, FIGS. 1 to 3 illustrate the treatment plan that is developed bythe treatment planning computer, freed of the constraints imposed bypreviously known apparatus. It now remains for the treatment controlcomputer of the radiotherapy apparatus to translate that treatment planinto a set of gantry moves, dose rates, and MLC moves.

Now, the minimum time in which each arc-segment can be delivered may bedefined by the dose or the gantry or any one of the leaves in the MLC.Thus:

Minimum dose time=inter-control point dose/Maximum dose rate,

Minimum gantry time=Distance of gantry move/Maximum gantry speed,

Minimum leaf time=Distance of leaf move/Maximum leaf speed,

-   -   (considered for each of the moving leaves)

The minimum time for the arc-segment is then the highest of all theseminima. This defines the time limiting parameter, which may be thegantry, dose or any of the 80 leaves.

If the dose is not the time limiting parameter, then the desired doserate can then be selected, being calculated as follows:

Desired dose rate=Control point dose/Minimum time

If the dose is the time limiting parameter, then the selected dose rateis of course the maximum dose rate.

The expected speeds of the Gantry and leaves can then be calculated fromthe selected dose rate as follows:

Expected arc-segment time=Control point dose/Selected dose rate

Expected gantry speed=Distance of gantry move/Expected arc-segment time

and for each of the leaves in the MLC:

Expected leaf speed=Distance of leaf move/Expected arc-segment time

FIG. 4 shows the choice between the dose rate and the gantry speed,ignoring the influence of MLC leaf speed for the purposes ofillustration. The x-axis is the dose rate per degree that is achieved,which corresponds to the cumulative dose delivered between two controlpoints. The solid line is the dose rate, while the dotted line is thegantry rotation speed. Both have maximum rates imposed by thelimitations of the specific apparatus being used. Thus, there is aspecific dose D per unit rotation that is achieved by the apparatusoperating at its maximum rotation speed and the maximum dose rate (perunit time).

To achieve a dose per unit rotation that is higher than D, the rotationspeed must be decreased in inverse proportion, and the rotation speed(dotted line) in this region therefore shows a 1/x profile while thedose rate (solid line) is steady. To achieve a dose rate per unitrotation lower than D, the dose rate must be reduced proportionately asshown.

FIG. 4 therefore illustrates the above calculations in a graphical form.

It should be noted that some radiotherapeutic apparatus do not actuallyallow a continuously variable dose rate. Instead, the dose rate is onlypermitted to adopt one of a number of preset levels. In such a case, thehighest available dose rate that is less than the desired dose rateshould be selected. The other factors can then be determined as above.

This is illustrated in FIG. 5. This corresponds to FIG. 4 except that inthe region of FIG. 4 where the dose rate is linear, the dose rate isforced to increase in steps up to the maximum dose rate. This iscompensated for by the rotation speed profile which adopts a series of1/x curves for each step, instead of simply for the maximum dose rate.Thus, the use of an apparatus without a continuously variable dose rateper unit time incurs a penalty in terms of the treatment time requiredbut not in terms of the dose distribution.

Ideally, the actual positions will be servoed to the actual delivereddose and therefore the actual speeds will vary slightly from theexpected speeds. However, the expected speed is a very useful parameterto ensure that the servos perform optimally.

In this way, a system is produced that is sufficiently close to ERDMLCin practice to mean that it can be treated as an ERDMLC system for mostpurposes.

FIG. 6 shows the relationships between the various computers involved inthe system. The treatment planning computer develops a treatment planwhich defines the treatment and passes this to a treatment controlcomputer. This determines, for each arc-segment, which factor is thetime-limiting factor and is thereby able to instruct each of the MLCcontrol computer, gantry control computer, and radiation controlcomputer as to the operation of their specific item during thatarc-segment.

In practice, it will be necessary to decide whether each illustratedcomputer should exist as a separate entity or whether some or all shouldbe combined into a single processor. This decision will depend on thepattern of expected computational load and the processing poweravailable.

Such a treatment plan can be implemented on a radiotherapy machine thatis substantially akin to those in current use. The physical differencescalled for by this invention lie in the control apparatus and thetreatment planning apparatus; the actual radiation head and the meansfor driving it, its MLC and other systems can be as those in currentuse. However, there are certain changes to the apparatus that could beuseful in the context of a machine operating in this manner.

First, the reeling system for the radiation head would benefit frombeing able to travel more than 360°, such as 2, 3 or more rotations.This would allow an operatir to treat 3 or more IMAT arcs withoutstopping, and also to image and treat the patient in a continuous arcfrom underneath.

Second, we propose to enclose the whole machine in a set of coverssimilar in style to a CT or MR machine, with the bore preferably closedoff at the inside end. Enclosing the moving parts removes thepossibility of a hazardous collision and therefore enables the speed ofthe gantry to be increased fairly easily from 1 RPM to at least 2 orpossibly up to 5 or 6, reducing the treatment times significantly.Increased speed also offers new options for Cone Beam image acquisitionfor example the images can be acquired during a single breath holdthereby eliminating any artefacts due to breathing motion.

Finally, to further reduce treatment times at high rotational speeds wepropose to remove the flattening filters that are normally placed in thepath of the beam in order to give a more uniform intensity of radiationacross the aperture of the device. These filters do of course act byreducing the intensity of the beam in the central area of the aperture,and therefore the compromise is between uniformity and overall dose. Anon-flat or non-uniform beam could instead be characterized andcompensated for in treatment planning. This would avoid difficultiesrelating to the non-uniformity of the beam intensity since adjustmentswould be made in the other treatment parameters, and would allow areduction in treatment time commensurate with the “recovery” ofradiation that was otherwise removed by the flattening filter.

Inertia Compensation

The above assumes that the elements of the apparatus on which theinvention is embodied are idealised geometry items—in other words, thatthey have no inertia and can therefore be manipulated at will withdesired changes to the speed taking effect immediately. In the realworld, this is not the case and geometry items have a distinct masswhich must be accelerated or decelerated to the desired speed.

If an assumption is made that a geometry item (i.e. a gantry arm, an MLCleaf, or any other part of the apparatus that affects the beam geometry)is inertia-less and that for reasons of efficiency always travels at itsmaximum design speed, then it follows that its position is predictablevia the trivial relationship distance=speed×time, or s=v_(max)·t wherev_(max) is the geometry item's maximum design speed.

Given that some time is required to accelerate the geometry item fromrest to that speed, there will be an initial period during which theactual speed of the item will be less than the theoretically expectedspeed. This translates into a lag in the item's position; other factorsaside, the actual position at any time would therefore be behind thepredicted position by an amount reflecting the item's inertia. That lagcan be corrected by continuing to accelerate the item beyond its designspeed so that it catches up; whilst this will require the item to exceedits maximum design speed, the excursion will be only brief and isacceptable. The “maximum” design speed is set at a maximum speed atwhich the item can safely move continuously for an extended period; themaximum safe transient speed will be higher.

This is shown in FIGS. 7 and 8. FIG. 7 shows the profile of velocitywith time. Initially (i.e. t=0), the item is at rest for the purpose ofthis example and v=0. The item then accelerates at a constantacceleration that is dictated by the power of whatever drives the itemand the inertial mass of the item. Thus, the velocity of the itemincreases smoothly at a constant slope until the point 10 at which itmeets the design maximum continuous velocity v_(max). The item isaccelerated further up to a peak velocity at 12 representing a maximumtransient velocity, after which it is decelerated to v_(max) which itreaches at point 14. Control of the acceleration and deceleration shouldbe by a suitable computation means to ensure that each geometry item isin the correct position with respect to the dose to be delivered. Thiswill mean that the areas 16 and 18 are substantially equal, in whichcase the lag caused by the acceleration period between t=0 and point 10will be exactly compensated for by the overspeed between points 10 and14.

The corresponding plot of distance with time is shown in FIG. 8. Ascompared to an idealised inertia-free system represented by the dottedline 20, the movement of an actual system (line 22) starts from rest andaccelerates, but is constantly behind the ideal line 20. After point 10,the slope of the distance-time curve equals that of the ideal system andthereafter becomes greater, enabling the actual system to catch up. Atpoint 14, the actual system has caught up with the ideal system and canthereafter proceed at the same speed.

This means that there is an initial period in which the position of anactual system is not easily predictable by a straightforwardrelationship such as s=v_(max)·t but is more complex. Thus, a propermodel of the behaviour of the item should take this into account.

An alternative approach takes note of the fact that after the catch-upperiod between t=0 and 14, the behaviour of the system is predictable.The point 14 at which catch-up is complete and easily predictablebehavious takes over is referred to as the “Inertia Compensation Point”and translates in FIG. 8 to a corresponding “Inertia CompensationDistance” 24 (ICD) and a corresponding “Inertia Compensation Time” 26(ICT). Provided that the distance or time being modelled is greater thanthe ICD or ICT respectively, we know that a simple s=v_(max)·t model canbe employed.

According to the treatment planning methodology set out above, for eachsegment a required time is calculated for each factor, i.e. (i) thegantry movement, (ii) the MLC movements, and (iii) the dose delivery,all at the maximum possible rate for each. The longest of these times isthen selected; this becomes the time for that segment, and the speeds orrates of the other two are scaled back so that they will take the sameoverall time for the segment.

To take inertia into account, those factors that have an inertia-relatedaspect to their modelling can first be modelled using a simple andstraightforward linear relationship, and a predicted time for thesegment obtained. In addition, for each segment the ICT is determinedand compared with this time. The greater of these is then used as therelevant time for that factor over the segment in the above comparison,to determine the rate limiting factor for that segment.

If that time thus chosen for the segment is an ICT, this will mean thatall relevant factors will be scaled back accordingly. Thenon-rate-limiting factors will be scaled back as previously, so thattheir total time for the segment corresponds to the segment time. Thespeed of the rate-limiting factor will then also be scaled back so thatit completes the segment at less than its maximum speed, at a lowerspeed that allows time for inertia compensation to take place.

There are a number of different ways in which the ICT can be determined.Ideally, it will be determined for each geometry item in each instanceby calculating the ICT for the particular instance concerned based onprior knowledge of the dynamics of the particular system. Otherexamples, such as a deceleration, acceleration from a non-zero speed tothe maximum speed, and acceleration to a speed less than the maximum arealso calculable using analogous approaches. Alternatively, it ispossible to measure the ICT of the specific example of accelerating ageometry item from rest to its maximum speed in the particular systemconcerned, and to adopt that as a basic ICT for the acceleration of theitem from rest to its maximum speed, i.e. ICT_(0-max). Other ICT valuescan then be determined by a straightforward scaling process applied toICT_(0-max). Thus, for a change in speed from Vi to V2, the ICT can thenbe approximated as:

${ICT}_{v_{1} - v_{2}} = {{ICT}_{0 - v_{\max}}\left( \frac{{v_{1} - v_{2}}}{v_{\max}} \right)}$

As a final alternative, given that all the ICTs are likely to berelatively small, a default ICT can be adopted (such as the ICT_(0-max)above) and used for all geometry items of that type in that system.Provided that a maximal ICT is adopted as the default, this will ensurethat a conservative approach is taken in which all geometry items areoffered sufficient time to compensate for inertia. It has the difficultythat where the actual ICT is less than the default ICT, the dose ratewill be scaled down more than is necessary, thereby lengthening thetreatment. In practice, however, the effect of this is likely to besmall whereas the use of a default ICT simplifies matters.

FIGS. 9 to 11 illustrate the inertia-compensated comparison processdiagrammatically. Each shows a bar chart representing the time requiredby each factor for a particular segment. The factors, as discussedabove, are the MLC traverse time (labelled “MLC”), the gantry rotationtime (labelled “Φ”) and the time required to deliver the specified doseat a maximum pulse repetition frequency (labelled “PRF”). On each graph,a dotted bar is also shown for the MLC and gantry rotations representingthe Inertia Compensation Time for that segment. This will of course bevariable, depending on the speed and position of the respective geometryitem at the start of the segment and the required speed (i.e. v_(max))and position at the end of the segment.

Thus, in the example shown in FIG. 9, a simple comparison between thetimes required by each factor shows the gantry angle Φ to be therate-determining step since, of the solid bars, Φ is the highest.Comparing the dotted bars representing the ICTs, the gantry angle isstill the rate determining step, but ICT_(Φ) is higher than thepredicted time. Thus, the speed of each factor for this segment willneed to be scaled down so that the segment time is equal to ICT_(Φ).This will ensure that, at the end of the segment, the gantry arm is inthe correct position.

FIG. 10 shows an example of a different segment. In this case, acomparison of the solid bars shows that the MLC traverse time is thelongest and therefore, using an ideal inertia-less system, this would bethe rate-determining factor. However, the Inertia Compensation Time forthe gantry angle (ICT_(Φ)) is greater than the time required for the MLCleaves to traverse, so in the comparison between the times required fordifferent factors, taking into account inertia compensation, it is thegantry angle factor which is the rate-determining step and the segmenttime is set to be equal to ICT_(Φ). Each factor will then be run at aspeed or dose rate so that the segment completes in this time.

FIG. 11 shows a comparison in which, without inertia compensation, theMLC traverse time would be the rate determining step. Taking intoaccount the ICTs for the MLC leaves and for the gantry angle, it isapparent that the ICT for the MLC leaves (ICT_(MLC)) is less than thepredicted time required for the MLC leaves to traverse at their maximumdesign speed. Therefore, in the comparison between factor times,ICT_(MLC) does not replace the predicted MLC traverse time. In addition,while ICT_(Φ), is greater than the predicted time for the gantry torotate (and is therefore the correct point of comparison in respect ofthe gantry angle), it is still less than the predicted MLC traversetime. Therefore, the rate-determining factor is the predicted MLCtraverse time, and in this segment the MLC leaves will be driven attheir maximum design speed and the other factors run at a speed so thatthey complete the segment in the time dictated by the MLC leaves.

It will of course be understood that many variations may be made to theabove-described embodiment without departing from the scope of thepresent invention.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A radiotherapeutic apparatus comprising a source able to emit a beamof therapeutic radiation along a beam axis, a multi-leaf collimatorarranged to collimate the beam to a desired shape, wherein the source isrotateable about a rotation axis that is substantially orthogonal to andintersects with the beam axis thereby to describe an arc around thataxis, further comprising a control means able to control the dose/timerate of the source, the rotation speed of the source, and the multi-leafcollimator position, the control means being arranged to receive atreatment plan in which the arc is divided into a plurality of notionalarc-segments, the treatment plan specifying the total dose for thearc-segment and a start and end MLC position, and to control the sourcein accordance with that plan over an arc-segment by calculating thetotal time required for the arc segment for a plurality of factorsincluding; i. an MLC leaf movement from a prescribed position at thestart of the arc-segment to a prescribed position at the end of thearc-segment, at a maximum leaf speed; ii. rotation of the source fromthe start to the end of the arc-segment at a maximum source rotationspeed; iii. delivery of the dose at a maximum dose rate per time;selecting the factor dictating the longest time, and controlling theapparatus so that the selected factor operates at its respective maximumand the remaining factors are operated at a reduced rate selected tomatch that longest time wherein the total time required for the arcsegment for at least one factor relating to a moving geometry item isthe greater of (a) a time required to complete the segment if thegeometry item traveled at a continuous defined upper speed for thegeometry item and (b) a time required to allow for adjustment of thespeed of the geometry item until it is travelling at the defined upperspeed.
 2. The radiotherapeutic apparatus according to claim 1 in whichthe time required to adjust the speed of the geometry item to thedefined upper speed is calculated from knowledge of the geometry itemstate and knowledge of its dynamic properties.
 3. The radiotherapeuticapparatus according to claim 1 in which the time required to adjust thespeed of the geometry item to the defined upper speed is calculated fromknowledge of an initial speed and an intended speed of the geometry itemby taking a proportion of a time to accelerate the geometry item fromrest to the defined upper speed, the proportion being substantiallyequal to the difference between the initial speed and the intendedspeed, divided by the defined upper speed.
 4. The radiotherapeuticapparatus according to claim 1 in which the time required to adjust thespeed of the geometry item to the defined upper speed is a preset amountrepresenting a time to accelerate the geometry item from rest to thatspeed.
 5. The radiotherapeutic apparatus according to claim 1 in whichthe time required to adjust the speed of the geometry item to thedefined upper speed includes a time to accelerate the geometry item tothat speed, and a further time to accelerate the geometry item beyondthat speed and subsequently decelerate it until travelling at thatspeed.
 6. The radiotherapeutic apparatus according to claim 1 whereinthe control means comprises a treatment control computer and anactuator.
 7. The radiotherapeutic apparatus according to claim 1 inwhich the radiation is not interrupted between the first arc-segment andthe second arc-segment.
 8. The radiotherapeutic apparatus according toclaim 1 arranged to monitor the dose actually delivered during atreatment and the actual position of the source, compare this to thetreatment plan, and servo the position of the source and or the doserate so that the actual relationship between delivered dose and sourceposition corresponds substantially to the treatment plan.
 9. Theradiotherapeutic apparatus according to claim 1 arranged to monitor thedose actually delivered during a treatment and the actual position ofthe MLC, compare this to the treatment plan, and servo the position ofthe MLC and or the dose rate so that the actual relationship betweendelivered dose and MLC position corresponds substantially to thetreatment plan.
 10. The radiotherapeutic apparatus according to claim 1arranged to monitor the dose actually delivered during a treatment andthe actual position of the patient positioning system, compare this tothe treatment plan, and servo the position of the patient positioningsystem and or the dose rate so that the actual relationship betweendelivered dose and patient positioning system position correspondssubstantially to the treatment plan.
 11. A treatment planning apparatus,for a radiotherapeutic apparatus of the type comprising (i) a sourceable to (a) emit a beam of therapeutic radiation along a beam axis and(b) rotate about a rotation axis that is substantially coincident withthe beam axis thereby to describe an arc around that axis, (ii) amulti-leaf collimator arranged to collimate the beam to a desired shape,and (iii) a control means able to control the dose rate of the source,the rotation of the source, and the multi-leaf collimator; the treatmentplanning apparatus being arranged to divide the arc into a plurality ofnotional arc-segments and to prepare a treatment plan which includes afirst arc-segment adapted to deliver a first specified dose during whichthe source rotates a first specified angle and the multi-leaf collimatorchanges shape at a first specified rate per degree, and a second arcsegment adapted to deliver a second specified dose during which thesource rotates a second specified angle and the multi-leaf collimatorchanges shape at a second specified rate per degree, such that at leastone of the first and second specified doses, the first and secondspecified angles, and the first and second specified rates per degreediffer as between the first and second arc-segments wherein the totaltime required for the arc segment for at least one factor relating to amoving geometry item is the greater of (a) a time required to completethe segment at a continuous defined upper speed for the geometry itemand (b) a time required to accelerate the geometry item until it istravelling at the defined upper speed.
 12. The treatment planningapparatus according to claim 11 in which the time required to adjust thespeed of the geometry item to the defined upper speed is calculated fromknowledge of the geometry item state and knowledge of its dynamicproperties.
 13. The treatment planning apparatus according to claim 11in which the time required to adjust the speed of the geometry item tothe defined upper speed is calculated from knowledge of an initial speedand an intended speed of the geometry item by taking a proportion of atime to accelerate the geometry item from rest to the defined upperspeed, the proportion being substantially equal to the differencebetween the initial speed and the intended speed, divided by the definedupper speed.
 14. The treatment planning apparatus according to claim 11in which the time required to adjust the speed of the geometry item tothe defined upper speed is a preset amount representing a time toaccelerate the geometry item from rest to that speed.
 15. The treatmentplanning apparatus according to claim 11 in which the time required toaccelerate the geometry item to the defined upper speed includes a timeto accelerate the geometry item to that speed, and a further time toaccelerate the geometry item beyond that speed and subsequentlydecelerate it until travelling at that speed.
 16. The treatment planningapparatus according to claim 11 in which the rotation speed and the doserate both constant during an arc-segment, and at least one thereof isdifferent as between the first arc-segment and the second arc-segment.17. The treatment planning apparatus according to claim 11 in which thefirst arc-segment and the second arc-segment are consecutive.
 18. Thetreatment planning apparatus according to claim 11, further comprisingan output means for transmitting the treatment plan to theradiotherapeutic apparatus.
 19. The treatment planning apparatusaccording to claim 11 arranged to calculate an irradiation time for eacharc-segment apt to deliver a required dose and to infer a rotation speedfrom the irradiation time.
 20. The treatment planning apparatusaccording to claim 11 arranged to prescribe a treatment plan thatincludes motion of a patient positioning system during the treatment ina manner correlated with motion of the source.
 21. The treatmentplanning apparatus according to claim 11 arranged to prescribe atreatment plan that includes motion of a patient positioning systemduring the treatment in a manner correlated with delivery of the dose.22. The treatment planning apparatus according to claim 11 in which thebeam axis and the axis of rotation of the source are substantiallyorthogonal.
 23. (canceled)